The kinetics of the interaction of colchicine with tubulin obtained from the brain membrane fraction are significantly different from those observed with cytoplasmic tubulin. Membrane components can similarly modify cytoplasmic tubulin. Detergent does not modify the interaction between colchicine and tubulin. The nature of the modification is not known. Under appropriate conditions c-AMP alters the rat of in vitro assembly of microtubules from crude supernatants. c-AMP has no effect on the assembly of purified tubulin into microtubules. An effect with c-GMP has not been demonstrated. How c-AMP changes the activity of tubulin has not been elucidated.